This invention relates to systems for processing television signals, and is more particularly concerned with systems for reducing noise in color video signals.
In the copending application Ser. No. 740,576, now U.S. Pat. No. 4,064,530 of Arthur Kaiser, James Kenneth Moore and William E. Glenn, Jr. entitled "Noise Reduction System for Color Television" filed Nov. 10, 1976 and assigned to the same assignee as the present invention, the disclosure of which is hereby incorporated by reference, there is described a system which is effective to reduce noise in both the luminance and chrominance components of a color television signal, even in the presence of significant motion between successive frames. The system utilizes frame store integration and includes a delay or storage device capable of storing a single television frame, a summing device for adding a fractional amplitude portion of the signal stored in the storage device to a fractional amplitude portion of the present, or incoming, video signal. The system functions as a recursive filter and is operative automatically to change the fractional amplitude portion of the stored signal fed back to the summing device as a function of the difference between stored and present signals thereby to change the integration time constant of the filter to accommodate for motion between the present signal and the stored frames. The present invention is directed to a feature of the system disclosed in the copending application; the following summary description of the noise reducing system will indicate the problem solved by the invention.
Referring to FIG. 1, which corresponds to FIG. 1 of the copending application, and considering a digital implementation of the noise reducing system, the video input signal on input line 10, which may be encoded by the pulse code modulation (PCM) technique described in U.S. Pat. No. 3,946,432 utilizing an 8-bit code, is applied via a variable attenuator 12 to one input terminal of a summing circuit 14. The output signal from the summer 14 is applied to a delay device 16 having a delay of substantially 525H; H represents one television line interval, which means that the delay device introduces a one frame delay since there are 525 lines per frame in the NTSC system. The output of delay device 16 is applied through a chroma inverter 18 and a variable attenuator 20 to a second input terminal of the summer 14. Attentuators 12 and 20, shown very schematically, are ganged and respectively introduce a transmission constant of (1 - a) and "a." That is, a fractional portion (1 - a) of the amplitude of the incoming or present video signal is applied as one input to the summer 14 and a fractional portion "a" of the amplitude of the stored video signal is applied to the other summer input. It will be evident that if the value of "a" is increased, for example, the proportion of the stored signal applied to the summer increases, and the proportion of the present video signal applied to the summer decreases. Conversely, if "a" is decreased, a larger proportion of the present signal and a smaller proportion of the stored signal are applied to the summer.
If there is no motion between successive frames, the video signals representing the successive frames will be identical in information content; only the amount of noise in each will differ. When a multiplicity of such identical signals are summed, in the manner just described, the result is a signal identical to any one of the summed signals and of the same magnitude as the incoming signal by virtue of the fact that the sum of "a" and (1 - a) is always unity. However, when random noise is present in the video signal, which may vary in amount and distribution from frame to frame, is summed, it tends to be canceled, or in any case is not reinforced as is the periodic video signal. However, motion between the incoming video signal and the stored frames affects the video picture sharpness and, in accordance with another aspect of the system described in the copending application, the motion problem is solved by detecting motion between stored frames and the present signal as the picture proceeds element-by-element through the system, and in response to the evaluation of such motion changes the value of the transmission factor "a" (and consequently (1 - a)) so as to alter the contribution of the stored past signals to the noise-reduced video output signal. If a picture element from the same scene object in the stored signals is sufficiently different in amplitude from the same element in the present video signal, the past history of that picture element is ignored and only the present signal is transmitted to the output terminal. Although there would be no signal-to-noise improvement for that particular picture element, it should be noted that, for the most part, motion is observed only on the boarders and in the fine detail of objects, and not on the broad areas of objects; that is, it is the interface between an object in a scene and its background that makes motion detectable in the displayed television picture. With this in mind, the system is operable in response to detected motion to alter "a" and (1 - a) in such a way as to accommodate motion, in the limit allowing "a" to go to zero, that is, to transmit only the present signal to the output terminal. In the implementation described in the aforementioned application the "pixel"-by-"pixel" comparison of the stored past frames with the corresponding "pixels" of the incoming video signal is performed at three times the color subcarrier frequency of the video signal, or 10.7 MHz in the NTSC system; thus, every 93 nanoseconds the system is called upon to make a judgment as to the amount of stored signal (that is, the proportion "a") that is to be fed back to the summer 14. By reasons of the high speed of operation and the nature of the comparison process, delays sometime occur in making the decision to make a change in the value of "a" which, in turn, produce certain undesirable artifacts in the displayed television picture.
The nature of this problem will be seen from an examination of FIG. 2 wherein the waveform A depicts an edge of a surface of an object in a scene in a static condition; that is, the various picture elements along a single line of the video signal are defined by the ascending leading edge of the pulse. Associated with the waveform A (which may be the stored signal) are upper and lower threshold levels, both designated V.sub.thresh, of predetermined magnitude below and above which, respectively, decisions are made respecting the pulse. If one desired to determine whether a difference exists between two signals, one stored and the other incoming, and there were no noise to be reckoned with, the threshold could take a variety of values, including practically zero. Unfortunately, significant noise is usually present and consequently, in order that the system not misleadingly recognize noise as motion, it is necessary to provide a threshold at some finite, compromise value. Waveform B (which may be regarded as the incoming video) represents the same picture points as waveform A, moved to the right with respect to waveform A, signifying that motion has occurred. For example, if waveform A occurs at time t.sub.O, waveform B may occur at (t.sub.O + .DELTA.t), which in the case of an NTSC television signal, .DELTA.t might be 1/30th of a second. Threshold levels of the same magnitude as these associated with waveform A are shown in association with waveform B.
To detect motion between the incoming signal B and the stored signal A, the signals are compared element-by-element, and applying the rule employed in the system of the copending application, whenever the difference between signals A and B exceeds the threshold level, the incoming signal B is switched to the output terminal, and, conversely, whenever the difference between signals A and B (it matters not which is the larger) is below the threshold, the stored signal A is transmitted to the output terminal along with a small amount of incoming signal. Waveform C in FIG. 1 is a plot of the difference between waveform A and waveform B, specifically, A minus B, and has the same arbitrary threshold levels associated therewith as in the case of waveforms A and B. Recalling that when the difference between the A and B waveforms exceeds the threshold level the system decides to use the incoming signal B, or some greater fraction thereof, the control function depicted by waveform D results. That is, starting at the left end of waveform C, so long as the value of the difference is less than the threshold level, signal A is to be coupled to the output with very little new video, and when the difference exceeds the threshold, at time T.sub.1, the system abruptly switches to signal B and continues to couple signal B to the output until time T.sub.2, at which time the difference signal falls below the threshold level, whereupon the signal A is again coupled to the output of the system. Thus, there is provided an on-off control function, which corresponds to the maximum feedback of stored signal and full bypass, respectively, in the system described in the copending application.
When the control function depicted by waveform D is applied to waveforms A and B, a signal having the waveform shown at E, which is not a true replica of what is desired, is coupled to the output of the system. Instead of getting only waveform B during the "up" portion of the control function waveform D during the period T.sub.1 to T.sub.2, that portion of the signal A which precedes T.sub.1 is coupled to the output, and, since the time T.sub.2 occurs before waveform B has reached the upper threshold level, less than all of the waveform B is coupled to the output, causing a discontinuity at the upper end of the waveform E. These "glitches" in the B signal transmitted to the output terminal cause distracting artifacts in the television picture, an unacceptable byproduct of the noise reducing function of the system.
Another problem inherent in the system as described in the aforementioned copending application is that the on-off control function causes the output of the system to consist of either a large fraction of the incoming video signal or a large fraction of the stored video signal. Since, in general, the stored signal will exhibit a much lower noise level than the incoming video, often more than 8 db lower, as the system switches from a large fraction of stored video to a large fraction of incoming video, or vice versa, a noise level discontinuity greater than 8 db is sometimes introduced in the output video signal which can be highly visible in the resultant television picture.
Thus, there are two basic problems associated with the control function D depicted in FIG. 1: first, the bypass action does not commence early enough to encompass the very start of the edge of waveform B, and it is turned off too soon to encompass the finish of the edge of the transmitted signal; and, it can cause abrupt noise level transitions to be introduced in the processed television picture. The primary object of the present invention is to overcome the above-outlined problems in noise-reducing systems of the integrating type and employing motion evaluation.